The present invention relates to lamp circuits and especially to a lamp bulb having multiple filaments driven one filament at a time and controlled to switch filaments upon the failure of a filament.
In U.S. waters, the Coast Guard requires that all off-shore obstructions, such as oil drilling and production platforms, be lighted at night by flashing lanterns which are placed so as to indicate the size and shape of the obstruction. The lanterns must flash synchronously at the rate of once per second, with power applied to lamp filaments for 0.3 second. The flash as seen by an observer must not appear as a point source, since the eye cannot range a point. The Coast Guard also will not permit use of xenon flash tubes for reasons which probably are due to the reaction of the eye to an abrupt flash of very short duration, i.e., it produces a sense of disorientation and cannot be ranged any better than a point.
This has led to use of a common type of lantern having six incandescent lamps mounted on a rotatable turret, so that if one burns out, another may be rotated into place, returning the lantern to service. The change of bulb is automatic, carried out by circuits which detect the broken filament and operate a small motor to rotate the turret.
Positioning of the new bulb is critical, its filament cannot be as much as 1/8 inch out of place and still have the lantern operate properly. This is because the bulb must be at the center of a Fresnel lens which converts the "point" of light from the filament to a column of light upon which an observer's eye can range. Many thousands are in use in U.S. waters and all over the world. They are, to a point, reliable and they meet Coast Guard requirements, such as being visible from a distance of five miles with the lantern at an elevation of twenty feet, and unattended operation for thirty (30) days.
Their disadvantages, however, are sufficient to warrant a complete redesign. They are poorly sealed against sea air, resulting in severe corrosion of the lamp system and control circuitry, making repairs expensive; they have a motor and moving parts, all of which can be eliminated by modern electronic design; and they are expensive.
The present invention is directed to a lamp having multiple filaments to overcome some of the disadvantages of presently used circuits and in which an electronic circuit detects a filament failure and switches to another, thus providing a reliable, longlasting flashing lamp of greatly reduced cost.